From: Shugo Maeda <redmine@...>
Date: 2012-02-24T14:48:56+09:00
Subject: [ruby-core:42854] [ruby-trunk - Feature #6073] Proposal of extending syntax of for loop


Issue #6073 has been updated by Shugo Maeda.


Hi,

Shouichi KAMIYA wrote:
> I propose to extend syntax of for loop which allows us to write multiple loop with guard easily.
> This extension is inspired by Scala. Here is a example

I'm interested in your idea.
# Feature #6070 was preparation for the same proposal.

> for i in 1..4 when i % 2 == 0
>     j in 5..8 when j % 4 == 0
>   puts "#{i}, #{j}"
> end
> 
> Above code is same as following code.
> 
> for i in 1..4
>   if i % 2 == 0
>     for j in 5..8
>       if j % 4 == 0
>         puts "#{i}, #{j}"
>       end
>     end
>   end
> end

Scala's for expression is more powerful.
If yield in used in a for expression, nested loop is converted to a combination of map/flatMap/withFilter.

For example, the following code:

  for z in (1..Float::INFINITY).lazy
      x in 1..z
      y in x..z
      when x**2 + y**2 == z**2
    yield [x, y, z]
  end

is equivalent to the following code:

  (1..Float::INFINITY).lazy.flat_map {|z|
    (1..z).flat_map {|x|
      (x..z).select {|y|
        x**2 + y**2 == z**2
      }.map {|y|
        [x, y, z]
      }
    }
  }

Please note that yield is already take in Ruby, so we need different Syntax.

What do you think of that feature?

----------------------------------------
Feature #6073: Proposal of extending syntax of for loop
https://bugs.ruby-lang.org/issues/6073

Author: Shouichi KAMIYA
Status: Open
Priority: Normal
Assignee: 
Category: 
Target version: 


Hi,

I propose to extend syntax of for loop which allows us to write multiple loop with guard easily.
This extension is inspired by Scala. Here is a example

for i in 1..4 when i % 2 == 0
    j in 5..8 when j % 4 == 0
  puts "#{i}, #{j}"
end

Above code is same as following code.

for i in 1..4
  if i % 2 == 0
    for j in 5..8
      if j % 4 == 0
        puts "#{i}, #{j}"
      end
    end
  end
end

I already implemented this syntax and attached a patch.
What do you think about this syntax guys?

Thank you,
Shouichi


-- 
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